1. Field of the Invention
The present invention relates to a thermally-assisted magnetic recording head that records information when the coercive force of a magnetic recording medium is decreased by irradiating near-field light onto the magnetic recording medium.
2. Description of the Related Art
In recent years, for magnetic recording devices such as magnetic disk devices, performance improvements of a thin film magnetic head and a magnetic recording medium are demanded in association with the high recording density. For the thin film magnetic head, a composite-type thin film magnetic head has widely been used, in which a reproducing head having a magneto resistive effect element (MR element) for reading and a magnetic recording head having an inductive-type electromagnetic transducer (a magnetic recording element) for writing are laminated on a substrate. In magnetic disk devices, the thin film magnetic head is disposed on a slider that flies just above a surface of the magnetic recording medium.
The magnetic recording medium is a discontinuous medium on which magnetic microparticles gather. Each of the magnetic microparticles has a single magnetic domain structure. Of the magnetic recording medium, one recording bit is configured with a plurality of magnetic microparticles. In order to increase the recording density, the asperity of a boundary of adjacent recording bits needs to be small. For this, the size of the magnetic microparticles needs to be decreased. However, when the size of the magnetic microparticles is decreased, thermal stability of the magnetization of the magnetic microparticles is also decreased corresponding to the decreased volume of the magnetic microparticles. In order to solve this problem, increasing the anisotropy energy of the magnetic microparticles is effective. However, when the anisotropy energy of the magnetic microparticles is increased, the coercive force of the magnetic recording medium is also increased. As a result, it becomes difficult to record information using a conventional magnetic recording head. The conventional magnetic recording head has such a drawback, and this is a large obstacle to achieving an increase in the recording density.
As a method to solve this problem, a so-called thermally-assisted magnetic recording method is proposed. In this method, a magnetic recording medium having a large coercive force is utilized. The magnetic field and heat are simultaneously added to a portion of the magnetic recording medium where information is recorded at the time of recording the information. Using this method, the information is recorded under a state where the temperature of the information record part is increased and the coercive force is decreased.
For a thermally-assisted magnetic recording, a method in which a laser light source is used to heat the magnetic recording medium is common. Such a method has two types of methods: one method is to heat the magnetic recording medium by guiding laser light to a recording unit via a waveguide, etc. (a direct heating type); and the other method is to heat the magnetic recording medium by converting the laser light to near-field light (a near-field light heating type). The near-field light is a type of electromagnetic field that is formed around a substance. Ordinary light cannot be tapered to a smaller region than its wavelength due to diffraction limitations. However, when light having an identical wavelength is irradiated onto a microstructure, near-field light that depends on the scale of the microstructure is generated, enabling the light to be tapered to a minimal region being only tens of nm in size.
In U.S. Patent Application Publication No. 2008/205202, another configuration is disclosed in which a near-field generator is disposed in a front part of a core of a waveguide through which light from a light emission element (a laser diode) propagates.
In U.S. Patent Application Publication No. 2008/151431, a configuration is disclosed in which a near-field generator plate and a near-field scatter plate are disposed in a front part of a waveguide in which the light enters and propagates. The near-field generator plate has a sharp edge part on one edge. The near-field scatter plate is arranged along an edge part that is on the opposite side of the sharp edge part of the near-field generator plate.
In Japanese Laid-Open Patent Publication No. 2009-070554, a configuration is disclosed in which a low refractive index part made of SiO2 is disposed between a near-field generator and a core of a waveguide in which light enters and propagates. A refractive index of the SiO2 that configures the low refractive index part is smaller than that of Ta2O5 that configures the core.
As a concrete method for generating the near-field light, a method using a so-called plasmon antenna is common. The plasmon antenna is a metal referred to as a near-field light probe that generates near-field light from a light-excited plasmon.
Direct irradiation of light generates the near-field light in the plasmon antenna. With this method, a conversion efficiency of converting irradiated light into the near-field light is low. Most of the energy of the irradiated light on the plasmon antenna reflects off the surface of the plasmon antenna or is converted into thermal energy. The size of the plasmon antenna is set to the wavelength of the light or less, so that the volume of the plasmon antenna is small. Accordingly, the temperature increase in the plasmon antenna due to the above-described heat generation becomes significantly large.
Due to the temperature increase, the volume of the plasmon antenna expands, and the plasmon antenna protrudes from an air bearing surface, which is a surface facing the magnetic recording medium. Then, the distance between an edge part of the air bearing surface of the MR element and the magnetic recording medium increases, and it becomes difficult to read servo signals recorded on the magnetic recording medium during the recording process. Moreover, when the heat generation is large, the plasmon antenna may melt.
Currently, a technology is proposed that does not directly irradiate light onto the plasmon antenna. For example, U.S. Pat. No. 7,330,404 discloses such a technology. In this technology, light propagating through a waveguide, such as an optical fiber, etc., is not directly irradiated onto the plasmon antenna, but the light is coupled to a plasmon generator in a surface plasmon mode via a buffer portion to excite the surface plasmon in the plasmon generator. The plasmon generator includes a near-field generator that is positioned on the air bearing surface and that generates the near-field light. At the interface between the waveguide and the buffer portion, the light propagating through the waveguide is completely reflected, and other light, which is referred to as evanescent light and which penetrates into the buffer portion, is simultaneously generated. The evanescent light and a collective oscillation of charges in the plasmon generator are coupled, and the surface plasmon in the plasmon generator is then excited. The excited surface plasmon propagates to the near-field generator along the plasmon generator, and then generates near-field light in the near-field generator. According to this technology, since the plasmon generator is not directly irradiated by the light propagating through the waveguide, an excessive temperature increase of the plasmon generator is prevented.
In thermally-assisted magnetic recording where a recording is performed while heating a predefined position of the magnetic recording medium, light from a light emission element (for example, a laser diode) is gradually tapered in the core of the waveguide such that a spot size is decreased, and the light then propagates to the near-field generator. However, according to results examined and analyzed by the present applicant, approximately 10% of the light emitted by the light emission element does not enter the core of the waveguide. Also, approximately 30% of the light entering the core is discharged to the outside when tapered in the core. Approximately 65% of the light tapered and propagating in the core is not coupled to the near-field generator. Light energy that does not contribute to generate the near-field light is converted to heat, and causes a temperature increase of the thermally-assisted magnetic recording head. Particularly, the light, which propagates in the core but is not coupled to the near-field generator, is highly energized due to being tapered in the core. Therefore, the light generates a large amount of heat.
As described above, the thermally-assisted magnetic recording has a problem in that performance and reliability of the thermally-assisted magnetic recording head deteriorate over time. For example, a thermal deformation gradually occurs in each part of the thermally-assisted magnetic recording head due to the temperature increase not only in the magnetic recording medium but also in the thermally-assisted magnetic recording head itself, and a magnetization is changed due to, for example, corrosion of the shield layer.
In U.S. Pat. No. 7,346,978, a configuration in which a metal layer is disposed to release heat of the inside of the thin film magnetic head is disclosed. With this configuration, Joule heat generated by a recording current flowing in a coil layer and heat due to eddy current generated in a core can be released. However, this configuration is not related to the thermally-assisted magnetic recording, and heat generated at and around the air bearing surface is not considered at all. Therefore, the temperature increase in a thermally-assisted magnetic recording head due to the thermally-assisted magnetic recording is hardly suppressed.